With the recent expansion of the applications of magnetic recording media, there has developed a need for magnetic recording media having good magnetic characteristics, particularly a high S/N ratio, especially in recent years with the increasing demand for better video and audio tapes.
To meet this demand, the magnetic particles in the magnetic coating must be uniformly dispersed and oriented in the binder thereof. Conventionally, the dispersibility of magnetic particles is improved and tape noise is reduced by; (1) producing finer magnetic particles; (2) adding a dispersant (surfactant or the like); (3) adding an agent to prevent cohesion of the magnetic particles (e.g., by adding solid particles) or (4) using a powerful dispersing device.
However, the present inventors have found that no matter how uniformly the magnetic particles are dispersed in the coating composition, they cohere during drying of the applied magnetic coating to thereby impair uniform dispersion of the magnetic particles. This problem can be avoided to some extent by increasing the viscosity of the magnetic coating composition or by adding an agent to prevent cohesion of the magnetic particles (e.g., by addition of solid particles), but there is a limit to the effectiveness of such methods since a highly viscous coating composition is difficult to apply and filter and because too large solid particles cannot be used.
A method is known for improving the orientation of the magnetic particles and providing better magnetic characteristics by passing the magnetic particles under a strong magnetic field and orienting them in the direction of coating application just after application and before evaporation of the solvent. But no matter how powerful the magnet is, it usually takes at least 10 seconds to evaporate the solvent, and, during this period the fluidity of the composition and free movability of the magnetic particles disturb the orientation of the magnetic particles, and in an extreme case, the orientation is completely lost. To prevent this, an even higher viscous magnetic coating composition may be prepared; however, then the magnetic particles cannot be oriented satisfactorily in a magnetic field having ordinary strength, and, if a stronger magnet is used, the surface roughness of the coating is increased to cause more noise and dropout. If the drying speed is increased, the smoothness of the surface of the coating is impaired by the drying air.